The Waif says to Arya:
"You, who walk in here with a coin you never earned, whose value you do not respect. Who are you?"
Was the usual method of being awarded a coin discussed in the books at any point? I don't recall. To me, it seems that she is questioning Jaqen's decision to do so, which does not seem her place. I thought Arya, for one so young especially, showed extraordinary resilience and resourcefulness/cleverness and that is what earned it for her. Is there usually a more rigid system for the distribution of these coins?